In recent years, water-absorbent resins having a high degree of water absorbency are developed and frequently used mainly for disposable uses, for example, as absorbent articles (e.g. disposable diapers and sanitary napkins) and further as water-retaining agents for agriculture and horticulture and for industrial sealing materials.
As to such water-absorbent resins, many monomers and hydrophilic polymers are proposed as their raw materials. Of them, acrylic water-absorbent resins as obtained from acrylic acid and/or its salt as the monomers are industrially most commonly used because of their high water absorbency (e.g. JP-A-054751/1987, JP-A-031306/1991, JP-A-211934/1994, U.S. Pat. Nos. 4,654,039, 5,338,810, 5,574,121, 5,562,646, EP 0574260, EP 0942014, U.S. Pat. Nos. 5,837,789, and 5,447,727).
Because water-absorbent resins are generally used for disposable uses (such as disposable diapers), it is essential that they are inexpensive. Therefore, the enhancement of their productivity is in high demand.
In addition, there is a natural high demand for the absorbent articles to avoid problems with respect to the safety and coloring of the absorbent articles. Specifically, the water-absorbent resin contains the unreacted residue of acrylic acid. Although the content of the unreacted acrylic acid is several hundred to about 1,000 ppm by weight (mass), a decrease in the content of the unreacted acrylic acid is demanded. In addition, the water-absorbent resin is combined with white pulp in the absorbent articles. Therefore there is a high demand for the water-absorbent resin also to be white so as not to give any foreign-substance feeling or appearance caused by coloring.
In addition, the water-absorbent resin is water-swellable and water-insoluble. However, in the water-absorbent resin, there is also contained an uncrosslinked water-soluble polymer (water-extractable component) in the range of several wt % to several tens of wt %. This water-extractable component has a bad influence upon the water absorption properties of the water-absorbent resin. Therefore, the decrease of the content of this water-extractable component is also demanded. Moreover, the absorbent articles containing the water-absorbent resin is required to possess acceptable water absorption properties under load (e.g. water absorption capacity under a load and liquid permeation quantity under a load).